Irreversible capacity generates at an initial charge of a lithium-ion secondary battery, due to a formation of a coated surface called SEI on a negative electrode. Capacity of the lithium-ion secondary battery decreases for this irreversible capacity.
Considering above, it is well-known that capacity of a lithium-ion secondary battery increases by doping (predoping) lithium to a negative electrode material for an irreversible capacity in advance (See Patent Article 1, for instance). However, as described in Patent Article 1, with a predoping method using a current collector with holes and electrochemically contacting a lithium metal foil with a negative electrode, there is a problem of a long predoping time which leads to a low productivity of a lithium-ion secondary battery and ununiform doping of lithium at negative electrode.
Therefore, a predoping technique wherein lithium metal powder, an electrode active material and a binding agent are mixed in a solvent is disclosed in Patent Article 2. However, with said Patent Article 2, due to a high reactivity of lithium metal powder, it is necessary to use a non-aqueous solvent as a solvent, and thus, a kind of usable a binding agent is limited. Furthermore, adhesion of an electrode is likely to decrease due to a residual lithium metal in the solvent. As a result, a cycle characteristic of secondary battery is likely to decrease. In addition, lithium metal deteriorates during predoping due to a high reactivity of the lithium metal, which leads to a long predoping time. Further, lithium metal powders migrate when applying and drying electrode material to a current collector and do not uniformly disperse in electrode; and internal resistance of a secondary battery increases.
In order to solve such problems, Patent Article 3 discloses a predoping technique wherein a stabilized lithium metal powder, in which said lithium metal powder is surface treated with carbon dioxide, an electrode active material and a binding agent are mixed in a solvent. However, not only a kind of usable binding agent is still limited, but lithium metal powder is still likely to deteriorate due to heating of a solvent removal. Further, migration of lithium metal powder, when applying and drying electrode material to a current collector, cannot be prevented and it is difficult to manufacture a uniform electrode.
Further, Patent Article 4 discloses a manufacturing method of capacitor electrode by a wet method using slurry including a lithium powder depositing polymer on its surface. With this method, due to a small particle diameter of lithium powder, the lithium powder eccentrically-locate on a surface of a granulated particle when preparing granulated particle by evaporating solvent from slurry including the lithium powder. Thus, the lithium powder cannot be uniformly placed in an electrode, which leads to a problem of lifetime characteristic. And shape forming is difficult when used for compression forming of an electrode.